1. Field of Invention
This invention relates generally to the field of electron beam circuit testers and particularly to apparatus adapted for testing large and very large scale integrated circuits.
2. Description of the Prior Art
As integrated circuits have become increasingly complex, traditional testing methods which involve the use of a mechanical probe to make electrical measurements on the surface of a circuit chip are no longer practical. Electron beam (e-beam) testing has become a practical alternative for testing large scale integrated circuits. Such testing equipment and techniques are described generally in E. Menzel and E. Kubalek, "Fundamentals of Electron Beam Testing of Integrated Circuits," Scanning, Vol. 5, pp. 103-122 (1983).
E-beam testing methods permit quantitative measurements of voltage waveforms from particular conductor paths of an integrated circuit to be obtained. In the typical test apparatus, a primary beam of electrons is focused on the point of interest on the surface of a specimen chip. Secondary electrons which are emitted in response to the impinging primary beam have an energy distribution which depends on the local surface conditions. The characteristic energy spectrum of the secondary electrons exhibits an energy shift as a voltage is applied to the irradiated portion of the specimen. The secondary electron energy spectrum can thus be analyzed to yield a measure of voltage at the surface of the chip analogous to that which would be obtained with a conventional probe applied to the circuit specimen. A spectrometer to perform such energy analysis is typically retrofitted to a conventional scanning electron microscope (SEM).
An alternative test method employs qualitative voltage contrast. In this technique, an image of an area of the specimen chip is obtained. Due to retarding electrical fields in the vicinity of positively biased regions of the chip surface, fewer secondary electrons will escape from these regions and they will therefore appear darker in the image. Conversely, regions of the chip surface with a negative electrical bias will appear brighter. Such images may be obtained with conventional SEMs.
Dynamic testing of circuits is facilitated by stroboscopic operation of the e-beam tester in synchronism with the operation of the device under test. Images and/or voltage measurements can thus be obtained at points in time corresponding to known logic states of the device.
Conventional SEMs have a relatively limited field of view. Systems with wider fields of view are hampered by distortion and loss of resolution towards the edge of the image. Consequently, voltage contrast imaging is difficult to accomplish over a significant portion of the surface of an integrated circuit without physically repositioning the specimen.
The imaging capability of systems adapted for quantitative waveform measurement is further hampered by the incorporation of an energy analyzer which impairs the field of view.
In addition, the distortions and aberrations of a conventional SEM make accurate beam positioning difficult. Highly accurate beam positioning is desirable so that the e-beam can be positioned on the surface of a specimen chip in response to positioning commands derived from information stored in computer aided design (CAD) systems.
Consequently, there is a need for an e-beam integrated circuit tester which offers a wide field of view within which the e-beam can be accurately positioned and which also incorporates an integral energy analyzer for performing quantitative waveform measurement.